Page 69 - Geology and Geochemistry of Oil and Gas

DEFORMATION OF ROCKS IN DEPTH 45
for the field development (compaction drive). Transitional zones may form at the
boundary, involving additional compaction in the reservoir rocks and under-
compaction in the caprocks.
Rocks are quite heterogeneous and are in an unstable state in the subsurface.
They are further destabilized because of non-uniform temperature distribution. Even
on considering formation temperature as resulting from the heat flow, and even if the
heat flow in the area is constant, the temperature of rocks will differ in different parts
of the area. Heat flow is related to the temperature drop as follows:

dT
q ¼ l (3.5)
l
dD
2
where q l ¼ the conductive heat flow (MW/m ), l ¼ the heat conductivity (W/m 1C),
dT and dD ¼ the temperature and depth differences (1C and m, respectively).
Heat conductivity depends not only on the properties of rocks, but also on their
oil- or water-saturation, temperature, and pressure. As an example, the heat
conductivity of dry rocks in Belorussia was as follows: clays — 1.02–1.56 W/m 1C;
sandstones — 1.34–2.12 W/m 1C; limestones — 1.67–3.44 W/m 1C (Sergiyenko,
1984). Consequently, the temperature of rocks also changes depending on the
properties of minerals composing the rock and on the water and oil saturations, with
all other variables being equal.
The above discussion shows that temperature affects the sealing properties of
rocks. Table 3.1 shows changes for the most reliable caprocks (Dobrynin and
Kuznetsov, 1993, p. 106). As the table indicates, the necessary caprock thickness
over the oil accumulations at the given temperatures is always higher than over the
gas accumulations. This is mainly due to different interfacial tensions at the oil/water
and gas/water contacts. Some improvement in sealing capacity at 601C as compared
to 401C is due to a decrease in pore diameter and, consequently, to a lower
permeability.
Interfacial tension also depends on temperature:

d ¼ d 0 ½1 oðT T 0 ފ (3.6)
where o is the temperature expansion coefficient (0.002 for the water) and (T T 0 ) is
the temperature difference (1C).
TABLE 3.1

Type of Dh reservoir ¼ f ðDh caprock Þ relationship for most reliable argillaceous caprocks of gas and oil
accumulations
T (1C) Accumulation Equation
40 Gas Dh reservoir ¼ 4Dh caprock
Oil Dh reservoir ¼ 17Dh caprock
60 Gas Dh reservoir ¼ 2Dh caprock
Oil Dh reservoir ¼ 7Dh caprock
Dh reservoir ¼ reservoir thickness; Dh caprock ¼ caprock thickness: